if you have a graphical desktop with a system tray conforming to the freedesktop recommendations (such as trayer, gnome-panel notification area, …) you can use nall to schedule a (almost) one-line script that will display the current song information. Once nall installed, if your shell-fm is run as a networked daemon, you just have to put the following script into ~/.nall/3listening :
$ cat ~/.nall/3listening #!/bin/sh HOST=192.168.0.1 PORT=54311 echo info | nc $HOST $PORT
this will tell nall to run the probe every 3 seconds, and to write 'listening:' as the tooltip title.
don't forget to replace HOST and PORT values regarding your configuration.
If your shell-fm is on your local machine, you can either write a script to write/read on the UNIX socket, or, (easier?) reading to a file written by shell-fm (configured to write-out the song title).